1. Field of the Invention
This invention relates generally to switch mode power supplies, and more particularly, to switch mode power supplies suitable for plasma processing that utilize feedforward control to prevent ripple and transients in the power supply output.
2. Brief Description of the Prior Art
The ability of a regulated power supply to prevent ripple and transients at the input from being transferred to the output can be improved by taking a signal proportional to the input voltage and combining it with the output from the closed-loop feedback control circuit in a way that counteracts the effect of changes in the input voltage. U.S. Pat. No. 6,359,799 discloses a three-phase power supply that uses feedforward to reduce ripple in the output. The optimal amount of a feedforward signal to be combined with the feedback signal varies with the operating conditions, and fairly elaborate control schemes such as those disclosed in U.S. Pat. Nos. 5,541,833 and 5,711,843 have been devised to adaptively adjust feedforward signals in a variety of industrial processes including plasma processing.
Feedforward techniques have been developed for use in pulse-width-modulated power supplies in which the voltage conversion ratio is determined by the switching duty cycle, such as those described in the publication by B. Arbetter, and D Maksimovic, “Feedforward pulse-width modulators for switching power converters,” IEEE Power Electronics Specialists Conference, June 1995, vol. 1, pp. 601-607. However, these techniques are not applicable to resonant power supplies. U.S. Pat. No. 6,049,473 utilizes a nonlinear variable-gain amplifier to adjust the small-signal gain of the feedforward signal path according to a pre-determined trajectory, but it lacks an adaptive feedforward scaling regulator that optimizes the amplitude of a feedforward signal based on measurements of the output of the power supply.
U.S. Pat. Nos. 5,535,906 and 6,697,265 disclose frequency-controlled resonant DC power supply circuits that are suitable for use in plasma processing. In typical implementations, they receive power from a three-phase-rectified DC bus that lacks bulk energy storage capacitors. The DC bus voltage obtained from unfiltered three-phase bridge rectifiers changes rapidly near the cusps where diode commutation occurs. The bandwidth of typical control loops for these power supplies is insufficient to compensate for the rapid changes in the bus voltage that occur near the commutation cusps, and this produces ripple peaks in the output of the power supply that occur with a repetition rate of six times the line frequency. Some plasma processes such as self induced plasma copper processes require lower values of ripple in the DC power than these types of power supplies can provide. The conversion ratio of these power supplies depends on the operating conditions as well as the operating frequency, so if feedforward compensation were to be used, it would need to be adaptive in nature.
It would be desirable if there were provided a simple and inexpensive adaptive feedforward circuit that minimizes perturbations in an output of a system that delivers power to a plasma process caused by periodic perturbations in a system input.